The tagging technique is the commonly known technique to execute imaging after modulating nuclear magnetization on an imaging target by applying a pre-saturation pulse and providing strip or lattice-like tags (magnetic markers) on an MRI image. The sequence for modulating nuclear magnetization of an imaging region is referred to as a tagging sequence. Imaging of, for example, myocardial distortion action can be executed by synchronizing the image acquisition sequence including the tagging sequence (hereinafter referred to as tagging imaging sequence) with an electrocardiographic waveform or pulse waveform of a living organism of an imaging target, obtaining and cine-displaying a plurality of images having different elapsed times from the synchronization timing (cine images).
The method for reconstructing an image from the data acquired by tagging imaging sequence is, for example, the SPAMM (Spatial Modulation of Magnetization) method (for example, refer to Patent Document 1) or the HARP method (for example, refer to Patent Document 2).
On the other hand, increased speed has been demanded in imaging procedure of MRI apparatuses. The conventional techniques for increasing speed are, for example, the parallel imaging method which saves imaging time by reducing the acquisition number of echo signals in the phase encode direction (for example, refer to Patent Document 3), the radial imaging method which saves imaging time by scanning a k-space in a radial pattern so as to reduce data sampling number of a high-frequency region, and the hybrid radial imaging method (for example, refer to Patent Document 4).    Patent Document 1: U.S. Pat. No. 5,054,489    Patent Document 2: WO 2000/09010    Patent Document 3: JP-A-2005-525185    Patent Document 4: JP-A-2004-344183
In the measurement data obtained by executing tagging imaging sequence, the echo signal for measurement and the higher harmonic component equivalent to a higher harmonic which is a pseudo-echo signal by a tagging sequence are generated. A higher harmonic is generated in the same direction as the gradient magnetic field for diphase to be applied during a tagging sequence. In this disclosure, the echo signal having the echo peak at the center of a k-space is referred to as a zero-order component (echo signal of zero-order). While on the other hand, higher harmonics of the echo signal generated by applying the pre-pulse of tagging are referred to as the first-order component and the second-order component of the echo signal or the first-order echo signal and the second-order echo signal, in order that the echo peak thereof is closer to the center of a k-space. In the imaging sequence wherein the pre-pulse of tagging is not applied, the echo signal having only the zero-order component is generated as measurement data.
In a tagging imaging sequence, higher harmonic components also need to be obtained in order to achieve high resolution of an image after being reconstructed and to maintain the tag with clarity. Especially, the first-order component of the echo having high signal intensity is extremely significant. Also in the HAPP method, a phase image is composed of the phase component of the image data obtained by Fourier transforming the first-order component of the echo signal, and the strain amount of a heart wall is derived and quantitatively evaluated. Therefore, highly accurate measurement of higher harmonic components is indispensable. In this manner, the first-order echo signal needs to be measured with high accuracy in a tagging imaging sequence. However, in the techniques for increasing speed such as parallel imaging method or hybrid radial imaging method, increased speed of imaging procedure is achieved by reducing (thinning) the number of data acquired from a predetermined region of a k-space. There is a possibility that the k-space region wherein the peak of the first-order component of the echo signal is generated may be included in the region from which the acquisition data is thinned out. Therefore, the first-order component of the echo signal cannot be measured with high accuracy when these speed-increasing techniques are applied to tagging imaging sequence, and acquisition of tags with clarity or quantitative evaluation cannot be executed appropriately.
In an aspect of this disclosure, there is provided an approach for applying the speed-increasing technique which thins out the measurement of high spatial frequency to the imaging technique which requires measurement of the first-order of the echo signal with high accuracy, without causing deterioration of image quality.
This disclosure presumes from the imaging condition the coordinate of a k-space wherein the peak of the first-order of an echo signal is generated, and controls scanning without reducing the data acquisition number of the region in the vicinity of the relevant coordinate.
In concrete terms, the disclosure provides a magnetic resonance imaging apparatus comprising;
static magnetic field generation means configured to generate a static magnetic field;
gradient magnetic field generation means configured to generate a gradient magnetic field in the directions of a plurality of axes;
high-frequency generation means configured to irradiate a high frequency magnetic field to an imaging target;
signal detection means configured to detect a nuclear magnetic resonance signal produced from the imaging target;
control means configured to execute a predetermined imaging pulse sequence by controlling operation of the gradient magnetic field generation means, high-frequency magnetic field generation means and the signal detection means so as to obtain k-space data; and
arithmetic processing means configured to perform arithmetic processing with respect to the k-space data so as to execute image reconstruction,
wherein:
the imaging pulse sequence includes a tagging sequence for modulating nuclear magnetization of the imaging target and imaging sequence for making a part of the k-space data not to be measured; and
the control means comprises measurement requiring region specification means for specifying the measurement region (measurement requiring region) required for reflecting the affect of the modulated nuclear magnetization on the image to be reconstructed and sequence modification means for modifying the relevant imaging sequence so as to measure the measurement region specified by the measurement requiring region specification means upon execution of the imaging sequence.
In accordance with the aforementioned approach, it is possible to apply the speed-increasing technique for thinning measurement of a high spatial frequency region to the imaging technique which requires the measurement of the first-order component of an echo signal with high accuracy, without causing deterioration of image quality.